Mean Turbulent Dissipation Rate Research Articles

Contrasting internal tide turbulence in a tributary of the Whittard Canyon

Submarine canyons that incise continental slopes are considered an important conduit for transport of suspended matter between shelf seas and deep-ocean. However, the exact mechanisms influencing matter transport and deposition are still largely unknown. We present moored observations of current flows, acoustic echo intensity and, in high-resolution, temperature variation from two contrasting mooring sites 6.5 km apart horizontally in 790 and 1130 m water depth in one of the tributaries incising the flank of the easternmost branch of Whittard Canyon, North-East Atlantic Ocean. The shallow site is situated on a steep thalweg slope which is supercritical for semidiurnal internal tides that have >100 m amplitudes and turbulent overturning evenly distributed over the 240 m range of observations, with no particular intensification near the seafloor. In contrast, the deep site is located on an along-canyon slope that is approximately critical for semidiurnal internal tides, with steeper slopes up-canyon and to the sidewalls. The internal tide amplitude at the deep site is about 75 m. In the lower 100 m, flow intensifies and becomes more rectilinear along the canyon axis, together with intensifying mean turbulence dissipation rate and tidally-averaged down-canyon flow. Over a semidiurnal tidal period three episodes of relatively large turbulence are observed. Turbulence is largest when an upslope moving bore passes. Smaller but still relatively large turbulence is associated with a secondary upslope bore and with strong convective overturning during the downslope tidal phase. The convection is clearly distinguished in temperature variance spectra between the buoyancy and Ozmidov frequencies, the range in which anisotropic stratified turbulence is expected. Averaged over the entire depth-time series, mean turbulent kinetic energy dissipation amounts to 4.4 ± 2 × 10-7 m2 s-3, equal for both sites. In the tributary, the first upslope moving bore and the convective overturning at the deep site will affect sediment resuspension most.

Spatiotemporal characteristics of atmospheric turbulence over China estimated using operational high-resolution soundings

Large-scale in situ observations are sorely lacking, leading to poor understanding of nationwide atmospheric turbulence over China. Nevertheless, high-resolution soundings have become available starting in 2011, providing a unique opportunity to investigate turbulence across China. Here, we calculated the mean turbulence dissipation rate (ϵ) from radiosonde measurements across China for the period 2011–2018 using Thorpe analysis. The atmospheric layers that had stronger turbulence indicated by larger ϵ generally came with larger Thorpe length but with smaller Brunt–Väisälä frequency. Overall, the clear-air ϵ in the free atmosphere exhibited large spatial variability with a ‘south-high north-low’ pattern. Large clear-air ϵ values were observed in both the lower stratosphere (LS) and upper troposphere (UT), especially over the Tibetan Plateau (TP) and its neighboring regions with complex terrain likely due to large-amplitude mountain waves. Particularly, less frequent but more intense clear-air turbulence was observed in both lower troposphere (LT) and UT over the TP, while more frequent, less intense clear-air turbulence was found in northern China. The all-sky turbulence considering the moist-saturation effects was much stronger in the troposphere, notably in southern China where convective clouds and precipitation oftentimes dominated. In the vertical direction, the altitude of peak clear-air ϵ in the troposphere was found to decrease poleward, broadly consistent with the meridional gradient of tropopause height in the Northern Hemisphere. A double-peak mode stood out for the profiles of clear-air ϵ at midlatitudes to the north of 30° N in winter: one peak was at altitudes of 15–18 km, and another at altitudes of 5–8 km. The strong shear instabilities around the westerly jet stream could account for the vertical bimodal structures. The seasonality of ϵ was also pronounced, reaching maxima in summer and minima in winter. Our results may help understand and avoid clear-air turbulence, as related to aviation safety among other issues.

EXPERIMENTAL INVESTIGATION OF TURBULENCE PRODUCED BY STATIONARY GRIDS

This work attempts to analyze the shear and normal stress components of a grid downstream under the influence of a rigid boundary. The correlation coefficient of Reynolds stresses is analyzed to inspect the fraction of turbulent motions contributing to fluctuating momentum flux. The mean turbulence dissipation rate is estimated downstream of the grid for three different mesh sizes and compared to a case without a grid. Based on the analysis, it is found that the flow approaches isotropy in the main flow region. In contrast, near the boundary, some residual inhomogeneity is observed, even at the far-field region of the grid. The Reynolds shear stress peak value decreases with the variation in mesh size of the grid. The negative and positive values of the stress depend on the spatial configuration in a vertical direction, occurrence of locations of the intersection of wires, and hole openings of different mesh sizes of the grid. The paper also analyzed the Kolmogorov complexity for various mesh sizes under the consideration of rigid boundaries.

Internal wave mixing in warming lake Grevelingen

Seasonal hypoxia or even anoxia can occur in some local deep basins of coastal waters. Such low summertime oxygen contents especially affect benthic life. The seasonal coastal hypoxia is commonly related to biological increased respiration and to physical limited vertical turbulent exchange that is associated with increased vertical stable density stratification. However, the same stratification can support internal waves that may break and locally generate turbulence. Here, we investigate the physics of internal wave motions in saltwater Lake Grevelingen (SW-Netherlands) during warming in mid-spring. Grevelingen is refreshed by weak tidal motions through an open sluice in its dam to the North Sea. The outer North Sea has a surface tidal range of about 3 m, but the lake surface tidal range is negligible (<0.1 m). To quantify vertical turbulent exchange, high-resolution temperature sensors were moored in conjunction with a current meter in 36 m water depth for three days. The site is known for anoxic conditions near the bottom in summer. While a 3-day, 30-m mean eddy diffusivity of <[Kz]> = 4 ± 2 × 10−5 m2 s−1 is found, the overall mean turbulence dissipation rate (∝turbulent flux) is <[ε]> = 1.1 ± 0.6 × 10−7 m2 s−3. Turbulent mixing occurs episodically, via near-surface cooling during night and increased winds, via sparse shear-driven breaking of internal waves at the main pycnocline, and via sheared near-bottom currents. Shear-driven turbulence is not commonly found in fresh-water lakes. Just below the main pycnocline around mid-depth a layer of weak turbulence is observed, as in fresh-water lakes. The observed turbulent exchange is sufficient to warm the near-bottom waters over the course of summer, but insufficient to prevent the biology from over-consuming oxygen contents.

Estimating the Mean Diapycnal Mixing Using a Finescale Strain Parameterization

AbstractFinescale methods are currently being applied to estimate the mean turbulent dissipation rate and diffusivity on regional and global scales. This study evaluates finescale estimates derived from isopycnal strain by comparing them with average microstructure profiles from six diverse environments including the equator, above ridges, near seamounts, and in strong currents. The finescale strain estimates are derived from at least 10 nearby Argo profiles (generally &lt;60 km distant) with no temporal restrictions, including measurements separated by seasons or decades. The absence of temporal limits is reasonable in these cases, since the authors find the dissipation rate is steady over seasonal time scales at the latitudes being considered (0°–30° and 40°–50°). In contrast, a seasonal cycle of a factor of 2–5 in the upper 1000 m is found under storm tracks (30°–40°) in both hemispheres. Agreement between the mean dissipation rate calculated using Argo profiles and mean from microstructure profiles is within a factor of 2–3 for 96% of the comparisons. This is both congruous with the physical scaling underlying the finescale parameterization and indicates that the method is effective for estimating the regional mean dissipation rates in the open ocean.

A laboratory realization of the Burgers' vortex cartoon of turbulence‐plankton interactions

AbstractA laboratory apparatus was constructed to physically realize the Burgers' vortex cartoon of turbulence‐plankton interactions. Fluid motion is induced by corotating two disks while simultaneously withdrawing fluid axially through hollow drive shafts. The technique creates a flow pattern that mimics a Burgers' vortex with size and strength consistent with turbulence vortices in the coastal and near‐surface zones. Specifically, the radius, circulation, and axial strain rate of the Burgers' vortex were specified to match typical dissipative vortices corresponding to two turbulence intensity levels (described by a mean turbulent dissipation rate of 0.009 cm2 s−3 and 0.096 cm2 s−3, respectively). Tomographic particle image velocimetry was used to quantify the flow field, calibrate the apparatus, verify that it produces the desired vortex characteristics, and provide a three‐dimensional velocity vector field to compare with zooplankton behavioral assays. The apparatus facilitates direct examination of the mechanistic aspects of plankton interaction with a turbulent‐like vortex, which is demonstrated via quantifying the swimming behavior of Acartia tonsa in and around the vortex flow structure.

Internal stresses and breakup of rigid isostatic aggregates in homogeneous and isotropic turbulence

AbstractBy characterising the hydrodynamic stresses generated by statistically homogeneous and isotropic turbulence in rigid aggregates, we estimate theoretically the rate of turbulent breakup of colloidal aggregates and the size distribution of the formed fragments. The adopted method combines direct numerical simulation of the turbulent field with a discrete element method based on Stokesian dynamics. In this way, not only is the mechanics of the aggregate modelled in detail, but the internal stresses are evaluated while the aggregate is moving in the turbulent flow. We examine doublets and cluster–cluster isostatic aggregates, where the failure of a single contact leads to the rupture of the aggregate and breakup occurs when the tensile force at a contact exceeds the cohesive strength of the bond. Owing to the different role of the internal stresses, the functional relationship between breakup frequency and turbulence dissipation rate is very different in the two cases. In the limit of very small and very large values, the frequency of breakup scales exponentially with the turbulence dissipation rate for doublets, while it follows a power law for cluster–cluster aggregates. For the case of large isostatic aggregates, it is confirmed that the proper scaling length for maximum stress and breakup is the radius of gyration. The cumulative fragment distribution function is nearly independent of the mean turbulence dissipation rate and can be approximated by the sum of a small erosive component and a term that is quadratic with respect to fragment size.

Resonant Enhancement of Turbulent Energy Dissipation

We periodically modulate a turbulent wind-tunnel flow with an active grid. We find a resonant enhancement of the mean turbulent dissipation rate at a modulation frequency which equals the large-eddy turnover rate. Thus, we find the best frequency to inject energy into a turbulent flow. The resonant response is characterized by the emergence of vortical structures in the flow and depends on the spatial mode of the stirring grid.